The fluid-pressure brake device for a vehicle of this type is described, for example, in JP 2012-20707 A. A fluid-pressure brake device for a vehicle described in FIG. 1 of JP 2012-20707 A comprise a master cylinder, a fluid-pressure control circuit, and a fluid-pressure control actuator (ABS modulator).
The master cylinder comprise, a cylinder body having a cylinder bore, an input piston assembled to the cylinder bore of the cylinder body so as to be movable in a cylinder axial direction, so that the input piston forms a reaction force fluid chamber capable of supplying and discharging a working fluid to and from an inside of the cylinder body, the input piston being integrally drivable with a brake operation member, and a master piston which is arranged coaxially with the input piston, and is assembled to the cylinder bore so as to be movable in the cylinder axial direction to form a drive fluid chamber capable of supplying and discharging the working fluid to and from the inside of the cylinder body and a pressure chamber capable of supplying and discharging the working fluid to and from the inside of the cylinder body, the master piston being driven by the input piston or the working fluid supplied to the drive fluid chamber, the master cylinder being configured so that a fluid pressure in the drive fluid chamber is cancelled out for the input piston.
The fluid-pressure control circuit comprise an electric fluid-pressure source and an electromagnetic valve, actions thereof being controlled by an electric control unit depending on an action amount of the brake operation member.
The electric fluid-pressure source can supply the working fluid to the reaction force fluid chamber and the drive fluid chamber.
The electromagnetic valve can independently control a fluid pressure in the reaction force fluid chamber and the fluid pressure in the drive fluid chamber.
The fluid-pressure control actuator is interposed on a master system fluid-pressure circuit which connects the pressure chamber of the master cylinder and a pressure chamber of a wheel cylinder with each other. An action of the fluid-pressure control actuator is controlled depending on a slip state of a wheel, and can control the fluid pressure supplied to the pressure chamber of the wheel cylinder.
The fluid-pressure brake device for a vehicle described in FIG. 1 of JP 2012-20707 A is configured so that, during a normal brake action, the input piston is driven against the fluid pressure in the reaction force fluid chamber, and the master piston is driven by the working fluid supplied to the drive fluid chamber. The working fluid required in the master system fluid-pressure circuit during the braking is acquired by a pressure reception effective diameter (hereinafter simply referred to as “pressure reception diameter”) and a stroke (a compression amount of the pressure chamber in the master cylinder) of the master piston.